Window Rough Opening Size Calculator
Calculate the correct rough opening size for any window.
Supports standard, egress, and custom windows in both metric and imperial.
A rough opening (RO) is the framed opening in a wall into which a window unit is installed. The rough opening must be larger than the window unit itself to allow for shimming, leveling, and insulation.
Why Rough Openings Are Larger Than Windows
When a window is manufactured, its dimensions refer to the overall frame size. The rough opening adds:
- Width: Add 50 mm (2") to the window width — 25 mm (1") on each side for shimming
- Height: Add 50–75 mm (2–3") to the window height — allows for shimming at the sill and a nailing flange at the top
The exact addition varies by manufacturer. Always check the window’s installation guide for the specified RO. If not available, the standard rule is:
Rough Opening Width = Window Width + 50 mm (2") Rough Opening Height = Window Height + 63 mm (2.5")
Egress Windows
For windows in bedrooms and basement sleeping areas, building codes in North America require minimum egress dimensions so occupants can escape in a fire. The International Residential Code (IRC) requires:
- Minimum net clear opening area: 5.7 ft² (0.53 m²)
- Minimum clear opening height: 24 inches (610 mm)
- Minimum clear opening width: 20 inches (508 mm)
- Maximum sill height from floor: 44 inches (1,118 mm)
Note: the “net clear opening” is the openable area of the window, not the frame size. This distinction fails more inspections than any other part of the rule. On a double-hung only the lower sash slides up, so the clear opening is a little under half the frame height, and plenty of windows that look generous miss the 5.7 ft² minimum outright. Casements do far better because the whole sash swings clear.
The egress check here uses the net clear opening figures you enter, taken from the manufacturer’s spec sheet. Leave those two fields blank and the check is skipped rather than guessed, because a guess against the frame size would tell you the window passes when it does not.
Framing Components
A window rough opening is framed with:
- King studs: Full-height studs on each side
- Jack studs (trimmers): Shorter studs supporting the header
- Header: Structural beam across the top of the opening
- Rough sill: Horizontal framing at the bottom of the opening
- Cripple studs: Short studs above and below the opening
This calculator gives the raw rough opening dimensions only.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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